The present invention relates to low frequency AM stereophonic broadcasting systems. Specifically, a receiving apparatus for demodulating a stereophonic broadcast including both amplitude modulated components, and angle modulated components is described.
Interest in providing stereophonic broadcast service to the low frequency radio spectrum presently used to broadcast monophonic information has recently developed. The Federal Communications Commission has opened a docket, FCC Docket No. 21313, "In The Matter of AM Stereophonic Broadcasting" for the purpose of evaluating numerous AM stereophonic broadcasting systems for use in the low frequency radio broadcast band.
One such system under consideration by the Federal Communications Commission includes a proposal by the Magnavox Consumer Electronics Company, wherein a system of stereophonic broadcasting is proposed for the low frequency broadcast spectrum. Left, L(T) and Right, R(T) stereophonically related signals are generated in a broadcast studio, and combined to generate a summation signal consisting of L(T)+R(T). The summation signal is used to amplitude modulate the radio frequency signal to provide a full carrier, double side band amplitude modulated signal which is received and demodulated by conventional monophonic AM receivers. Additionally, the system provides for linearly phase modulating the broadcast signal carrier with a difference signal consisting of L(T)-R(T). Further, the Magnavox proposal calls for modulating the carrier frequency with a low frequency identification signal at a modulating index different from the linear phase modulation index.
In U.S. Pat. No. 4,302,626, a type of superheterodyne receiver is described for demodulating such a signal. The use of superheterodyne techniques however has some disadvantages in that preselection circuitry of the receiver must be frequency tuned in synchronization with the tuning of the local oscillator. Certain design problems are encountered, known to those skilled in the art, when precise tracking between the preselection circuitry and the local oscillator signal frequency is required. In stereophonic receivers, mistuning between the local oscillator and preselection circuitry can cause distortion to one or the other sideband sets which are located on each side of the carrier signal. Distortion of the carrier signal sidebands degrades receiver performance.
Additionally, phase detection of the angle modulated components at the intermediate frequency level in the superheterodyne receiver is accomplished in the aforesaid referenced patent by limiting the intermediate frequency signal, and then detecting the phase of the intermediate frequency signal. As is known to those skilled in the art, many phase detectors detect at the time the excursion of the amplitude limited signal level instantaneously crosses through a zero amplitude level. Time differences between the zero crossings are used to derive phase information.
When automobile radios are used, typically, the intermediate frequency signal is at 260 kHz and the number of samples used, i.e., the number of zero crossings, determines the amount of filtering required in deriving the phase information. The recovered phase information necessary includes a minor amount of intermediate frequency signal as a distortion product. Those skilled in the art will recognize that higher intermediate frequencies are easier to filter and requires fewer poles of filtering than lower frequency signals to obtain the same information to distortion product level. Thus, it has occurred to the inventor that detection of phase changes at the radio frequency rate, rather than at a lower intermediate frequency rate, will prove beneficial in providing increased sampling of the phase modulation improving the quality of the recovered difference signal.